various special-type recorders in all price ranges. The electrical characteris- 
tics and working mechanisms of these recorders are listed in the extremely 
useful Recorder Manual by Aronson (1962). The inexpensive Amprobe instrument is 
a@ pressure-sensitive, paper type, with a lift-plate stylus, and with a 5-second 
action and chart speeds up to 15 inches per hour. The 50-microampere model used 
in these circuits has a 7400-ohm meter resistance. 
Ohm's Law 
Although the voltage and currents involved are of low value, it is important 
to have a basic understanding of electrical resistance so that the meter mechanism 
of the recorder will not be damaged when setting up the circuits. Ohm's law is 
used to determine the value of the resistances in the meter circuit. 
Ohm's law, in common terms, states that the current in amperes (I) is equal 
to the pressure in volts (E) divided by the resistance (R) in ohms; that is: 
l= 
TO| ts 
» or transposed, 
ty 
R= 7, or E =IR. 
In a series circuit the total resistance is the same as the individual 
resistance, i.e., 
Rn = Ry ae R, ate Ra et cetera. 
Thus, using a 1.4-volt mercury battery, the current that would flow through 
a microammeter recorder with a 7400-ohm resistance is: 
fies ag 
Te 7hOO ohms = 0.000189 amp. 
6 
Since 1 microampere (ua) = 1/1,000,000 of an ampere, then 0.000189 X 10° = 
189 pa. As the maximum deflection of the meter is only 50 na, it is easily seen 
that the recorder in series with a 1.4 battery would be damaged by drawing more 
than three times its rated capacity. 
If we insert a resistor (for example 30,0CO ohms) in series with the meter 
resistance, we decrease the current across the meter coil (fig. 1A). We then 
have a series curcuit where: 
ae _ 1-4 V__ ft ~ 0.0000374 amp. x 10° oi Gua 
T= 3B 3700 ome te eae 
The current flowing in the circuit is thus 37.4 pa, or 12.6 ya below the maximum 
allowable meter current of 50 na. 
If resistors are connected in parallel, then the total resistance is less 
than the lowest value of resistance in the parallel circuit. It is desirable to 
shunt across the meter so that Ry (fig. 1B) may be used as a control resistor, 
leaving the shunt to divert some of the excessive current from the meter. This, 
in effect, makes a series-parallel circuit. R, is the series control resistor, 
and Ro a 200-ohm shunt resistor; along with R32, the meter resistance forms the 
parallel circuit. 
ae ee 
